In the prior art, there is a huge number of documents disclosing various kinds of organopolysiloxanes, which polymers can alternatively be referred to as silicones. Silicone compositions form cured products having a lot of desirable properties like weather and heat resistance, hydrophobicity and rubber-like properties such as hardness and elongation. According to their intended use the sterical structure as well as the substitution pattern of the organopolysiloxanes can be varied. The sterical structure predominantly depends on the presence of and the relation between M, D, T and/or Q units, whereas the substitution pattern determines the curing mechanism and a lot of further properties.
For example, WO 2004/107458 A2 discloses a light emitting device (LED) encapsulating composition, which comprises at least one polyorganosiloxane with an average compositional formula:(R1R2R3SiO1/2)M·(R4R5SiO2/2)D·(R6SiO3/2)T·(SiO4/2)Q,wherein T+Q>0; R1 to R6 are identical or different radicals selected from organic groups, a hydroxyl group and a hydrogen atom; at least one of R1 to R6 is either a hydrocarbon group with a multiple bond or a hydrogen atom, and at least one of R1 to R6 is an aromatic group. The composition can be used for the encapsulation of LEDs which emit light in the blue through UV spectrum.
WO 2004/148812 A1 discloses a curable silicone composition comprising one linear and one branched organopolysiloxane each having alkenyl groups and one organopolysiloxane having silicon hydrogen groups, all polysiloxanes comprising phenyl groups. The composition is characterized by good fillability and curability. A similar organopolysiloxane composition is disclosed in WO 2008/023537 A1. Both compositions exhibit good optical properties.
Further silicone compositions comprising aryl substituents and being curable by hydrosilylation are disclosed in US 20070073026 A1 and US 2004/0178509 A1.
Aryl groups are a good example for demonstrating the dependency of certain properties on the substituents of the organopolysiloxane. On the one hand, aryl groups may enlarge the refractive index of a silicone composition which for many applications may be highly desired. However, the presence of aryl groups in silicone compositions decreases the thermal stability and UV stability of the cured silicone body, which limits its applicability for instance in high power lighting devices like in headlamps of automobiles.
Thus, cured silicone compositions having a normal refractive index advantageously show better thermal stability and UV stability than those having a high refractive index. This might be due to the fact that in such silicones methyl groups are commonly present as organic groups beside reactive groups like alkenyl and Si—H groups. On the other hand, without any aryl groups, such cured silicone compositions show bad barrier properties and corrosive gases and humidity can pass through the sealing material. As a result, substrates might be corrupted and their lifetime may be significantly decreased.